The present invention relates generally to airfoils, and more particularly to an airfoil having a variable cross section to provide the alterable lift characteristics.
Airplane wings provide an aircraft with lift and drag characteristics. During takeoff and landing, the wing must provide high lift at low speeds. To maintain a desired altitude, the wing must provide the same amount of lift at high speeds. The camber line of the wing is the mean line between the upper and lower wing surfaces. The chord line is a straight line between the leading and trailing edge of the wing. By increasing the camber line above the chord line, the lift of the wing is increased. By moving the camber line below the chord line, the lift becomes negative.
Conventionally, flaps are used to increase low lift speed. Flaps, when raised and lowered, generate a turbulence in the flow of air over the wing. Small gaps between the wing and flap add to the discontinuity. This is commonly known as a "dirty" configuration. The "dirty" configuration creates drag and makes the airfoil inefficient, increasing power requirements and using more fuel. Thus, maintaining a smooth flow over the wing surface would reduce turbulence and drag.
Several attempts have been made to reduce the drag associated with a wing by controlling the surface of the airfoil. One example is described in U.S. Pat. No. 4,247,066 in which a central jack screw is connected to several beam members. As the jack screw is rotated, the curvature of the wing is varied. One disadvantage to such a design is that the complex structure is believed to provide a significant weight penalty. Also, the wing is believed to be difficult to assemble.
Another U.S. Pat. No. 4,341,176 describes a reversible camber airfoil. This design is particularly suited for use in a sailboat application. The airfoil design comprises a plurality of rigid spars that connect the upper portion and lower portion of a flexible and movable skin. This design is not suitable for airplanes since there is no mechanism for changing the camber line of the airfoil. Changing the camber line allows an airplane wing to generate the desired high lift.
Another example of an adjustable airfoil is described in U.S. Pat. No. 4,582,278. This airfoil uses a thin secondary skin applied to the top primary skin of the aircraft surface. Pressurized fluid is introduced into the cavity between the primary skin and the secondary skin which causes the secondary skin to separate from the primary skin to cause a rounding effect on the upper surface of the airfoil. One problem associated with such a wing design is that the sealing the wing to provide the desired characteristics is believed to be unreliable.
U.S. Pat. No. 3,986,688 describes an airfoil having a variable configuration. The mechanism described controls the thickness of the leading edge of the wing. Hydraulic actuators coupled to a linkage mechanism are used to separate the upper surface and lower surface of the wing. One disadvantage to such an invention is that the trailing edge of the wing is not varied.
While various designs have been proposed in the art, manufacturability, weight and reliablility are disadvantages of many of the designs.